1. Field of the Invention
The present invention relates to piezoelectric devices including a piezoelectric thin film and methods for manufacturing such piezoelectric devices.
2. Description of the Related Art
Piezoelectric devices including a piezoelectric thin film are currently being developed. A plurality of methods for manufacturing a piezoelectric thin film are available to form such piezoelectric devices. For example, as disclosed in Japanese Unexamined Patent Application Publication No. 2002-534886, a method for manufacturing a piezoelectric device is developed in which a piezoelectric thin film is split off from a piezoelectric substrate along an ion-implanted layer.
The method for manufacturing a piezoelectric device in which a piezoelectric thin film is split off from a piezoelectric substrate along an ion-implanted layer will now be described.
FIG. 1 is a series of sectional views schematically showing a process for manufacturing a piezoelectric device in Japanese Unexamined Patent Application Publication No. 2002-534886. Referring first to FIG. 1A, hydrogen ions are implanted into a surface 7 of a piezoelectric substrate 5 to form an ion-implanted layer 6 at a predetermined depth d in the piezoelectric substrate 5. Referring now to FIG. 1B, a bonding material 8 is deposited on the surface 7 of the piezoelectric substrate 5 by sputtering. Referring now to FIG. 1C, a support substrate 9 is bonded to the piezoelectric substrate 5. Finally, the stack of the piezoelectric substrate 5 and the support substrate 9 is heated to split the piezoelectric substrate 5 along the ion-implanted layer 6. As a result, a piezoelectric thin film 4 shown in FIG. 1D is formed on the support substrate 9. In the method of manufacture in Japanese Unexamined Patent Application Publication No. 2002-534886, the split surface of the piezoelectric thin film 4 is planarized by polishing.
Thereafter, electrodes are formed on the piezoelectric thin film 4 shown in FIG. 1D to obtain a piezoelectric device such as an SAW device. The bonding material 8 is, for example, SiO2 or a metal material (such as Cu, Ti, or Al).
Al and Al alloys based on Al, which have low electrical resistivity and low specific gravity, are commonly used as electrode materials for piezoelectric devices. Al electrodes, however, have poor stress migration resistance; a high electric power supplied to Al electrodes causes a hillock or void in the Al electrodes and finally shorts or breaks the Al electrodes, thus damaging the piezoelectric device.
To solve this problem, for example, Japanese Unexamined Patent Application Publication No. 2002-305425 discloses a method for forming an Al electrode with improved power durability etching a surface of a piezoelectric substrate 2 and then epitaxially growing Al on the piezoelectric substrate 2. Specifically, a several-nm-thick damaged layer 3 (see FIG. 2A) formed on the surface of the piezoelectric substrate 2 by, for example, polishing is initially etched with a strong acid such as hydrofluoric acid to expose crystal planes 14 on the +Z axis side of the piezoelectric substrate 2 on which epitaxial growth is possible. As shown in FIG. 2B, after the removal of the damaged layer 3 by etching, an extremely fine stepped structure is exposed in the surface of the piezoelectric substrate 2. This structure has flat surfaces formed by the crystal planes 14 on the +Z axis side of the piezoelectric substrate 2 (hereinafter abbreviated as “+Z planes 14”) perpendicular to the +Z axis, which is a crystal axis. Al is then epitaxially grown on the +Z planes 14 of the piezoelectric substrate 2 in the +Z axis direction such that the crystal growth plane thereof is parallel to the +Z planes 14. Thus, as shown in FIG. 2C, an Al electrode 11 whose crystal orientation is aligned in the +Z axis direction of the piezoelectric substrate 2 is formed on the piezoelectric substrate 2, which improves the power durability of the Al electrode.
The method in Japanese Unexamined Patent Application Publication No. 2002-305425 above, however, has the problem of an extremely low etching rate in the +Y axis direction of the piezoelectric substrate 2. If the piezoelectric thin film 4 shown in FIG. 1D is used to provide a high-performance piezoelectric device with improved power durability, the etchant concentration or the etching time needs to be increased to expose the +Z planes of the piezoelectric thin film 4 on which epitaxial growth is possible. Increasing the etchant concentration or the etching time, however, causes a problem in that the bonding material 8 between the piezoelectric thin film 4 and the support substrate 9 dissolves in the strong acid.